This invention relates to well logging methods and apparatus, and more particularly, to methods and apparatus for producing measurements from multiple transducer arrays and combining them to provide compensation for variations in instrumentation and borehole conditions.
It is well known in the art of acoustic logging that some degree of compensation for variations in travel time introduced by changes in borehole diameter may be provided by a borehole tool that includes two receivers and one transmitter. (Such arrays will be abbreviated hereinafter by using a "T" to represent a transmitter and an "R" to represent a receiver with the relative positions at the T's and R's indicated by the sequence, the hyphen "-" separating the transducers indicating a common signal path. Thus a T-RR array specifies a transmitter on one side of two receivers, with the receivers having in common the signal path between the transmitter and its nearest receiver.) Unfortunately, such a T-RR arrangement does not compensate for the tilt of the tool with respect to the axis of the borehole. To overcome the tilt problem an additional transmitter may be provided to form a tool that has a T-RR-T array. As described in U.S. Pat. No. 3,257,639 issued to F. P. Kokesh on June 21, 1966, each of the two transmitters may be selectively operated and the travel time to each of the two receivers measured. The individual travel time measurements may then be combined to produce an average travel time for the interval between the two receivers. That average time has the advantage of being compensated for both changes in borehole diameter and tilt of the tool.
As with many different types of measurements under conditions varying non-homogeneously in a direction radial to the borehole, acoustic measurements appear to vary with distance between transmitter and receiver or, more appropriately for two-receiver arrays, with the distance between the transmitter and a point midway between the receivers. It is for this reason that the borehole-compensating type tools have two transmitters located equidistant from that mid-point.
As recognized in U.S. Pat. No. 3,312,934 issued Apr. 4, 1967 to A. A. Stripling, one reason why the acoustic velocity may vary with different T-R distances is that different signal paths may result with the longer distance having a path somewhat farther from the borehole and deeper into the formation. This deeper path may be less affected by factors which radially alter acoustic properties when drilled or exposed to the borehole fluid, such as hydrophilic shales which tend to swell. This altered zone may exist deep enough into the formation to cause a short T-R distance to measure, at least in part, properties representative of this altered zone, rather than the desired unaltered formation. Longer T-R distances, such as 8 or 10 feet, are preferred to overcome this particular formation alteration problem.
Longer T-R distances require longer tools, and in the older two-receiver type arrays, i.e., those of the T-RR type, an increase from 3 to 8 feet in T-R distance requires a 5 foot longer tool. However, in the T-RR-T borehole compensating tools, such a substantial increase in T-R distance results in undesirably long tools since the T-R distance occurs twice. Longer tools are undesirable since their length makes them more expensive and difficult to transport and increases the problem associated with getting them down crooked or inclined boreholes.
It is therefore an object of this invention to provide method and apparatus which retain both the advantages of long T-R distances and borehole compensation without requiring unduly long borehole tools.
Borehole compensating type arrays are also employed in sidewall devices such as disclosed in U.S. Pat. No. 3,849,721, issued to T. J. Calvert on Nov. 19, 1974. Here longer T-R distances in the prior art T-RR-T array increase skid length, which unfortunately decreases the chances of keeping the skid in continuous contact with the borehole wall.
It is an additional object of the present invention to provide methods and apparatus which retain the borehole compensation features provided by a T-RR-T array in a sidewall skid, yet allow for increasing the T-R distance without increasing the skid length.
Previous approaches to providing at least a partial compensating system without unduly long tool lengths, such as described in U.S. Pat. No. 3,207,256 issued to R. B. Blizard on Sept. 2, 1965 or U.S. Pat. No. 3,330,374 issued to D. E. Broussard et al. on July 11, 1967, require memorization of at least two different measurements for at least two different distances. This requirement leads to additional memory costs and more vulnerability to depth positioning problems such as introduced by a yo-yo motion of the tool. Further, the compensation for tool tilt is not always complete.
Therefore, it is a further object of this invention to provide methods and apparatus which provide a more complete borehole compensation including tool tilt yet require a minimum of different depth positions and memorization distances.
A further problem with either the T-RR-T or its reciprocal, The R-TT-R array, is that because of the large distances between the T's in the T-RR-T array or the R's in the R-TT-R array, the operating conditions for transducers located at the ends of the array may be quite different, resulting in significant differences in the received signals which are presumed to be equal. For example, if severe tool tilt places one of the outer transducers in a substantially eccentered position while the like corresponding transducer at the other end of the tool remains more centered, signals associated with these outer transducers could vary considerably and, in turn, could affect both the travel time and the amplitude measurements.
It is therefore an object of the present invention to provide methods and apparatus having all like transducers grouped together while still providing borehole compensated measurements.
When a T is between a pair of R's or an R between a pair of T's, there is often a problem with electrical noise, as for example with electrical cross-talk from an electrically noisy transmitter circuit into receiver leads which must pass close to the transmitter or still worse, from a transmitter firing lead having high voltage and current transients, as in the case of acoustic tools, which pass by one of the receivers or received signal amplifiers. For example, the firing pulse leads going to the bottom transmitter in the T-RR-T array must pass by both receivers. A further appreciation of the electrical and mechanical problems introduced by transmitter leads passing receivers may be found in U.S. Pat. Nos. 3,734,233 and 3,712,410. It would be highly desirable to have a compensating array where all receivers could be isolated from all transmitters and further, where no high voltage pulse leads pass anywhere near a receiver, its associated amplifier or receiver signal lines.
Therefore, it is a further object of the present invention to provide a borehole compensating type array where all receivers and associated receiver signal circuitry may be readily isolated from the transmitters and their associated firing circuitry.
In prior art compensation type arrays and in some two receiver arrays, it was not possible to obtain measurements over some parts of the borehole. For example, in the T-RR-T array, the tool might not operate properly with the upper transmitter inside the casing and the remaining transmitter and both receivers still out in the open hole. Similarly, measurements of the formation could not be made in the critical bottom part of the hole for a distance corresponding to at least one transmitter-receiver distance. It would be desirable to be able to log as close to the bottom as possible, even if it were necessary to temporarily forego the compensation feature for this interval.
It is therefore an additional object of the present invention to provide methods and apparatus which are capable of making measurements over its entire array length.
In acoustic tools which may be required to operate at some distance from the borehole wall, the acoustic signals arriving at a given receiver effectively leave the borehole wall at a point ahead of the receiver, the displacement of the point varying with the approach direction. This gives rise to what is known as a refraction error. This error and one correction technique for compensating type arrays, as described in U.S. Pat. No. 3,304,536, issued Feb. 14, 1967 to F. P. Kokesh, and U.S. Pat. No. 3,524,162, issued Aug. 11, 1970 to F. W. Zill, involves the use of an additional receiver with each of the two existing receivers. Each additional receiver is spaced from each existing receiver by a small distance corresponding approximately to twice the displacement introduced by the refraction error--one displacement for each of the two different reception directions. Thus, four receivers are used, two for each reception direction.
Further, in the T-RR-T type array, omnidirectional receivers are required since each receiver must anticipate signals arriving from either the upper or lower transmitter. Highly desirable directional receivers cannot be used unless four receivers are employed, as in the above refraction correction approach; i.e., two receivers directed towards each of the transmitters. Another approach would be to use the R-TT-R type array, but now desirable directional transmitters cannot be used unless, as with the four-receiver approach, four transmitters are employed. Needless to say, these extra transmitters add considerable control complexity and expense.
Consequently, it is an object of this invention to provide a compensation type array which permits all receivers and transmitters to be directional, yet still requires only four transducers to produce the measurements needed to provide compensation.
Two different T-R distance investigations are desirable and, as described in the above discussed U.S. Pat. No. 3,312,934 patent, it is possible the close comparison of the different investigations may even lead to a direct indication of the presence of hydrocarbons when it occurs in the form of gas, or in some cases, to estimate the degree of shaliness as suggested in U.S. Pat. No. 3,096,502, issued to C. B. Vogel on July 2, 1963. It should be apparent that in order for measurements having different T-R distances to be useful in these applications, the measurements must be as accurate as possible. The accuracy of the T-RR type of measurements illustrated in the above patents is often such that the observed differences in these different investigations may actually be due to uncompensated tilt or system measurement errors, rather than radial differences in the acoustic properties of the formations. At least two measurements are required for this application, and it is important that both of these measurements be borehole compensated.
It is therefore an additional object of the present invention to provide methods and apparatus to obtain two differently spaced, that is, long and short T-R, investigations that are both compensated for borehole and system measurement errors.
When using prior art compensation type arrays to obtain the different T-R distance investigations, two additional outside transducers at an additional distance beyond those usually provided and a large number of additional measurement subcycles beyond the four normally employed would be necessary. Furthermore, the tool length would be increased by twice the desired difference in distance. Such requirements for additional transducers and tool length render the second measurement impractical under many circumstances, since the second measurement is usually redundant to the first measurement. However, if it could be provided without such costly complications, this second measurement would increase the value of the primary measurement by providing substantial assurance that at least the longer T-R distance was adequate for altered formations and when favorable conditions did occur, would provide a direct indication of the presence of gas.
It is therefore a still further object of the present invention to provide methods and apparatus to obtain simultaneously two different borehole compensated measurements with different T-R investigation distances and without requiring additional transducers, substantial increases in tool length or a significant number of additional measurement subcycles.